Long optical waveguide transmission links are used in optical transmission technology. Production dictates that the optical waveguides are not completely isotropic, but rather weakly birefringent. The long transmission link results in frequency-dependent polarization transformation, called polarization mode dispersion or polarization dispersion, abbreviated to PMD. Through the change in the polarization of the optical signal as a function of the optical frequency and, associated therewith, different frequency-dependent delays, this PMD leads to the widening of transmitted pulses. As such, at the receiving end, the identifiability of the pulses is reduced and, as a result, the transmitted data rate is limited. The term “principal states of polarization”, referred to as PSP below, designates those two states of polarization which are orthogonal to one another and to a first approximation do not change when the optical frequency changes. In polarization-maintaining optical waveguides, the principal states of polarization coincide with the principal axes; in other words, are horizontal and vertical. In general, however, the principal states of polarization are arbitrary orthogonal pairs of elliptic states of polarization. The principal states of polarization have different group delays, whose difference is referred to as “differential group delay”, DGD below. If an optical signal is transmitted with one principal state of polarization, then, to a first-order approximation, no pulse widening takes place. If it is transmitted with a polarization which, in the case of splitting according to the two principal states of polarization, corresponds to power components that are identical there, maximum pulse widening occurs because two pulses of identical strength, with delay differences equal to DGD, are superposed. If the principal states of polarization change as a function of the optical frequency, then it is the case, however, that, when a principal state of polarization which corresponds to a specific frequency is used on the input side, the output state of polarization will nevertheless change as a function of the frequency, but actually only in a higher order than the first order. This is referred to as higher-order PMD. Higher-order PMD generally occurs, although first-order PMD is predominant due to its effects and, therefore, must be compensated preferentially. This is aggravated by the fact that the transmission response of the link, and hence the PMD too, changes as a result of temperature change or mechanical stress. Therefore, use is made of adaptive PMD compensators which are inserted in the transmission path. To drive these compensators, PMD distortions must be detected in the optical receiver. The compensator can then be set optimally using a gradient algorithm, for example.
In Electronic Letters, Feb. 17, 1994, Volume 30, no. 4, pages 348 to 349, use is made of a bandpass filter for filtering a data signal whose PMD is to be detected. A power detector at the filter output supplies a signal which is higher, the smaller the PMD distortions are. In Electron. Lett. 34(1998) 23, pages 2258 to 2259, use is made of a combination of a number of bandpass filters with downstream power detectors, in which case, instead of individual signals, it is also possible to use a linear combination of the signals. By using bandpass filters having different center frequencies, it becomes possible, at the same time, to detect even relatively large PMD distortions which exceed, e.g., a bit duration of the signal. However, bandpass filters are poorly suited to monolithic integration; for example, in Si or SiGe. Moreover, unavoidable group delay distortions in the bandpass filters have the result that optimal PMD detection and hence equalization is not possible.
In Proceedings OEC 94, 14e-12, pages 258 to 259, Makuhari Trade Fair, Japan 1994, a different method is used, in which the power of the differential signal between decision-circuit output and decision-circuit input is evaluated. Incorrect decisions may occur, however, particularly in the event of severe PMD distortions in which the DGD exceeds the bit duration, so that the signal obtained in such cases is an unsuitable criterion for the presence of PMD distortions.
An object of the present invention, therefore, is to specify a reliable detector even for relatively large values of the differential group delay which can be integrated in a simple manner and, unlike bandpass filters, is not subject to intrinsic distortions through group delay distortions.